Increasing demands on the world's natural oil supplies have recently tended to limit the quantities of oil which is made available to manufacturers and oil consumers generally. This has caused great concern in the transformer industry which uses substantial quantities of oil annually to provide the liquid coolant for liquid-cooled transformers.
The patent to Montsinger U.S. Pat. No. 2,036,068 issued Mar. 31, 1936 suggests the replacement of a portion of the liquid coolant for a transformer by spheres of fired clay. The Montsinger approach falls short of commercial promise because fired clay has a specific gravity of more than twice that of the oil which it replaces. Consequently, the use of fired clay as described by Montsinger would substantially increase the shipping costs of the coolant material, the finished structure, and could necessitate redesign of various support structures for the transformer.
In accordance with applicant's invention, a material heretofore used as a thermal insulating medium is used with a reduced amount of liquid coolant, and yet provides effective thermal transfer from a transformer. In this regard, if the amount of liquid coolant in the transformer tank were reduced, and then combined with a thermal insulating material, it would be expected that the total thermal transfer capability would be dramatically reduced. However, in accordance with the present invention, the thermal transfer capability of the liquid coolant is synergistically enhanced by the thermal insulating medium.
The thermal insulating medium used as a coolant displacement material in accordance with the teachings of this invention is preferably a plurality of glass spheres having one or more closed voids. Such spheres are widely known as an insulating medium, for example, see the patent to Alford U.S. Pat. No. 3,086,898 dated Apr. 23, 1963, the patent to Veatch et al. U.S. Pat. No. 2,797,201 issued June 25, 1957, the patent to Veatch et al. U.S. Pat. No. 2,978,340 issued Apr. 4, 1961, and the patent to Veatch et al. U.S. Pat. No. 3,030,215 issued Apr. 17, 1962.
In accordance with the principles of this invention, the glass spheres have a specific gravity which is greater than the specific gravity of oil but substantially less than the specific gravity of solid glass material to minimize shipping costs and yet provide for self-location of the glass spheres. The glass spheres preferably have a diameter which is sufficiently large to provide flow passages of the dimension needed for proper coolant flow, but are sufficiently small so that the glass spheres can occupy most available locations in the transformer and transformer tank and provide a substantial venturi effect. The glass spheres have a wall thickness at their exterior surface which insures the mechanical integrity of the spheres during pouring of the spheres into the tank. The glass material is selected to be substantially inert in the coolant environment and is further selected to have good dielectric characteristics to avoid dielectric breakdown.